1. Field of the Invention
This invention relates to a speech synthesis method and apparatus for synthesizing excitation signals by a synthesis filter for producing a synthesized speech signal.
2. Description of the Related Art
In a speech synthesis apparatus employing a synthesis filter, it has been practiced to use a post-filter placed directly after the speech synthesis filter for improving subjective quality of the speech signal.
As such post filter, there is known one having characteristics of emphasizing the spectrum of the synthesized speech obtained by a synthesis filter. This spectrum emphasizing effect may be realized by connecting a filter having characteristics corresponding to blunted frequency characteristics of the synthesis filter, that is a filter having characteristics proximate to flat characteristics, in tandem with a synthesis filter.
FIG. 1 schematically shows the structure of a speech synthesis device employing an LPC synthesis filter 102 performing speech synthesis by exploiting linear predictive coding (LPC). In FIG. 1, an excitation signal ex(n) and LPC coefficients {.alpha.(i)} (i=1, 2, . . . , N) are supplied to input terminals 101, 106, respectively. The LPC synthesis filter 102 filters the excitation signal ex(n)to produce a synthesized speech signal s1(n). The transfer function 1/A(z) of the LPC synthesis filter 102 may be represented, by the supplied LPC coefficients {.alpha.(i)}, in accordance with the equation (1): ##EQU1##
The synthesized speech signal s1(n) is sent to a spectrum emphasizing filter 103 for spectrum emphasis and taken out as a speech signal s2(n) at an output terminal 104.
With the spectrum emphasizing filter 103, operating as a conventional post-filter, the poles of the transfer function of the LPC synthesis filter 102 are shifted radially towards the origin (0) for producing a transfer function having characteristics corresponding to frequency characteristics of the synthesis filter. If only the denominator is processed, tilt of low range emphasis is left, so the blunted characteristics are applied to the numerator by way of tilt adjustment, in accordance with the following equation (2): ##EQU2##
However, if spectrum emphasis is performed using a filter having characteristics as shown in the equation (2), the coefficients gn, gd are difficult to set, while it is difficult to accommodate frequency characteristics or the psychoacoustic hearing feeling, such that, if proper coefficients are not set, the sound quality becomes worse. There is also a problem that, since the spectrum emphasizing characteristics are determined solely by these two coefficients gn and gd, the degree of freedom in setting the spectrum emphasizing characteristics is lowered.